Re: [IRCA] Wobblers and a possible explanation

["Craig Healy" <bubba@xxxxxxxxxxxxxxxxxxxx>]

> Charles A Taylor, WD4INP wrote:
>
> I see a possible causes for the Wobblers.
>
> It's common among tube-type transmitters that, when the oscillator tube
> reaches the end of its useful life (when the transconductance drops below
X
> micro-mhos), the inherent feedback inside the transmitter will cause the
> transmitter to "take off" and oscillate at some frequency near the
assigned
> frequency. The station will sound about normal, but the carrier frequency
> will be a few kHz off and "wobbulate," or vary with a frequency-modulated
> characteristic caused mainly by bass components of the audio modulation.

That makes sense.  Around 1975 I worked for WGNG-550 (now WDDZ).  They had
an RCA BTA-1R tube transmitter, 1960 vintage.  I think it was that
oscillator tube that got tired.  The station was on the air, had voltage and
current readings about where they should be, and even antenna base current
about normal.  Modulation monitor was normal, too.  Problem was, the station
had migrated down to 480KHz or so, out of range of any radio I had at that
time.  I wasn't on duty, but the engineer on that shift fixed it.

It is remarkable that an ostensibly narrow-band transmitter would function
that far off it's tuned frequency.  But it did.  I don't recall any
wobbulations leading up to that episode, but every transmitter reacts
differently to failures.

Speaking of oscillators..  If a station were to intentionally shift it's
frequency from 10Hz high to 10Hz low in some recognizable pattern, could the
Spectran-type programs detect that?  I would think that would be relatively
easy to do, and would be an interesting variant on a DX test.  Slow speed
morse code with very narrow band frequency shift keying.  I would think a
small battery-operated flip flop circuit using an ancient 555 timer chip
could make the keying.  In the common Harris Gates 1 transmitter, there's
probably a place where it could be tied in through a switching diode and
capacitor to minimally jump the frequency a small (and legal) amount.  This
would probably be totally transparent on any normal radio.  I'll have to
look into this, if I ever get any free time.

Craig Healy
Providence, RI

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